GRR is NRR without the upgrades, no customer counts for more than they paid twelve months ago, so it can never exceed 100 %.
GRR = sum of min(MRR today, MRR 12 months ago) per customer ÷ MRR 12 months ago
GRR shows how much of the base you keep before anybody upgrades. A company with 110 % NRR and 80 % GRR loses a fifth of its base every year and makes up for it with a few growing accounts.
sumwerk shows GRR next to NRR for every month.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.